More information on Standard Shackles:

Standard screw type shackles have near endless applications. They are typically used for halyards, blocks, travelers or other locations that do not require frequent removal.

Tylaska's halyard shackles have the highest strength-to-weight ratio of any existing halyard shackle on the market. A captive 17-4PH stainless pin makes for trouble-free operation. The all-metal machined line thimble allows for use as a 2-1 purchase without the wear or melting that occurs with plastic molded thimbles.

Antal’s Twin Halyard Block is designed specifically for use with a 2:1 main halyard. The block features an extremely high working load in relationship to its size. The block is hardly larger, and weighs little more, than a standard main halyard shackle with comparable working load.

Harken, Selden, and Ronstan manufacture a huge selection of bow, D, twist, slotted and pin shackles in a wide variety of sizes. You can find their shackles in options of forged or stamped. Forged shackles are generally stronger and maintain higher working loads. RaceLite makes halyard shackles for wire halyards.